The present invention generally relates to vehicle braking and, more particularly, to an anti-lock brake control system.
Many automotive vehicles are equipped with an anti-lock brake system (ABS) which modulates the brake force actuation to control the amount of tire slip between the tire and the road surface, in order to maintain stability of the vehicle during braking. Conventional brake systems generate a braking force at the wheels of the vehicle, in response to the vehicle operator depressing a brake pedal, which, in turn, resists rotation of the wheels and creates a deceleration force at the tire and driving surface interface. If the tire slips beyond a certain amount, the braking force between the tire and the driving surface is controlled to reduce the tire slip on the driving surface so as to maintain vehicle stability during vehicle braking. As each tire approaches or exceeds the peak of a mu-slip curve, electronic control is employed to maintain lateral force generating potential. As the tire slip increases past the peak on the slip curve, the wheels may lock up, thereby creating possible vehicle instability.
Typical anti-lock brake systems prevent the wheels from locking by reducing the brake force applied to the wheels by modulating the brake force. As a consequence, the brake force is repeatedly increased and decreased in a cyclical fashion. In order to maintain stability of the vehicle, most anti-lock brake systems do not maximize vehicle braking. Instead, current anti-lock brake control systems limit tire slip to the detriment of decreased stopping distances, in case the driver should command a change in direction (i.e. yaw) as occurs when the steering wheel is turned.
In addition, the capacity of vehicle brake systems is generally limited by the amount of vehicle weight that can be allocated to the brake system both from a mass and volume standpoint. Brake components such as the rotor, temporarily store thermal energy that is generated by the sliding friction between the brake pads and the rotor when slip occurs during an anti-lock braking event. The energy transformed by the brake components is generally proportional to the brake torque multiplied by the number of revolutions of the wheel. As the number of wheel revolutions is reduced for a given type of stop, the amount of energy temporarily absorbed by the brakes is likewise reduced. Many tire constructions exhibit a longitudinal force versus slip curve that only gradually reduces the available longitudinal force as the tire slip is increased past the peak of the mu-slip curve. In cases where repeated vehicle stops with minimum fade are required, the amount of thermal energy that builds up in the brake components may lead to relatively high brake component temperature and eventually to reduced braking capability. This reduced braking capability could easily be less than the effect of slipping the tire at close to the 100% slip point with cooler brake operation.
Accordingly, there is a need for an anti-lock brake control system in a vehicle that controls vehicle braking so as to limit the thermal load applied to brake components to prevent brake component over-temperature and maintain adequate vehicle braking capabilities during repeated brake events occurring before the brake components have had a sufficient time to cool.
In accordance with the teachings of the present invention, an anti-lock brake control system is provided for a vehicle having a wheel and a brake for applying braking force to the wheel in response to an operator brake command input. The brake control system includes an operator input for commanding vehicle braking, and a brake actuator for applying braking force to the wheel in response to the operator input. In addition, the brake control system includes a temperature determining device., such as a temperature sensor, for sensing temperature of a brake related component, and a wheel speed sensor for sensing rotational speed of the wheel. Further, the brake control system includes a controller for controlling the amount of braking force applied by the brake actuator in accordance with tire slip as determined by the wheel speed. The controller determines the tire slip during braking and increases the tire slip to limit brake heat generation while maintaining adequate braking when the determined temperature exceeds a predetermined temperature threshold. Accordingly, the brake control system of the present invention reduces thermal load at the brake related component when the thermal load is excessive.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.